In the Vanderboren Vault, there is an Iron Cobra construct guardian. Is such a thing thematically appropriate for such brave and experienced adventurers as the Vanderborens? It strikes me as not impressive enough for them, but still too powerful for level 1 adventurers.

So my question is this, should I replace it with a Dreadguard from the MM2?

This is the eternal problem of the Lab Milieu in DnD. First level characters need first level challenges - which often does not make sense. Tenth level characters rarely fight first level challenges - that would be booring.

This is more and more becoming my main problem with DnD in all its incarnations.

Assuming the party have the signet ring, you can do whatever you think is powerful enough. You could make the guardian something heavier (Shield Guardian, Brass Golem, 10 Iron Cobras) and have it charge then stop 5' away from the nearest party member. Let them work out that it is 25' away from the person with the signet ring or whatever.

The players assume you know what you're doing and will rationalise it better than any mere DM. No reason not to (ab)use that once in a while... :)

This is more and more becoming my main problem with DnD in all its incarnations.

Same here my freind, I like playing in a big sandbox world. If you go into the ancient wyrms lair at first level...you should expect death. Why would anyone put in a guardian that only first level types could overcome? It's giving up logic at the altar of fetishizing game balance.

But what Carborundum said is a good way to make it work. I have had players go up against things they have no way of defeating, and that's because they can't with combat, they have to either avoid it or use ingenuity.

The way I looked at it, the Vanderborens were generally nice people, and didn't want to scatter lethal traps everywhere.

The iron cobra is a compromise - its poison reservoir can be filled with non-lethal venom, so it can capture (rather than just flat-out kill) any intruders. And it (rather than a bigger nastier construct) was chosen because of its capacity for stealth. Remember that while when the PCs get there, the vault was a big open empty space, the Vanderborens probably expected it to be full of chests, souvenirs, great big sculptures and paintings, and other curiosities from their adventures. Lots of cover for something small and sneaky like an iron cobra, but conversely not the sort of place where you'd want a stone golem stomping around in (doesn't help you much if the golem kills your would-be thief but crushes all your valuables in the process!)

Good point. You could make it an expensive version with an extra +15 to hit for say 1d2+1 and non-lethal poison for something like 3d10 damage DC 30. Just to scare the crap out of the players by saying...

Of course, you need to be the kind of DM who is happy making stuff up without justifying it too much with the rules. They are only guidelines after all.

With the thing being so goshdarned scary, you could let everyone roll spot checks and just tell the guy with the highest that it's avoiding or extra attentive of the player with the ring (or Lavinia). They'll figure it out, and keep a healthy respect for the protection of the family jewels. They'll also immediately suspect the robber had a ring, since no-one could handle that kind of poison for long.

Assuming they subdue it with the ring and not with large hammers, you could let them hear a few clanks in other walls when it stands down. It's buddies... It all ends up pretty easy for them to feel proud of themselves for figuring it all out, and that will keep them from asking too many questions about the 'fairness' of the encounter. It may even save their lives later since they know that some places may well NOT be CR-appropriate.

Players feel tough, and clever. DM feels that the guardian was appropriate yet secretly knows it was fair. Players reckon they've a badass DM who doesn't pull punches, which combines with their self-esteem to make a top session. And a rep as a badass lets a DM get away with all sorts of stuff in the name of a good game.

You could also see the cobra as an extra precaution. After all, the treasure was already defended very cleverly, and it was in a vault under castle Teraknian. Then it is more logical that the CR is not too high.

I can actually se4e an in-game reason for the low security. Security costs, and a burglar that broke into the bank would probably not be much trouble by any personal security the Vanderborens could afford. So, what they're after are confidence tricksters... who presumably would be bad at fighting, and thus the Iron Cobra would suffice.

of course, due to the DnD lab milieu, a good confidence trickster is also a decent fighter...

When Verrick and Larissa Vanderboren gained their vault under Castle Teraknian, they were owed a favour by a master construct maker named Arthur Morannin (CG Gnome wizard XVIII) for capturing a working Kolyarut for him, and so he agreed to make for them a guardian. Larissa wanted a mindless golem or iron cobra, but Verrick instead chose to have a nimblewright created and contributed some of the XP for its creation. Because he so liked the personality of the construct created, Verrick named it Maximilian Vanderboren and treated it as his firstborn son. He often spent time with it between adventures, and though many intelligent constructs would have come to loathe this servile, menial existence. Verrick lavished enough attention upon Maximilian that he in turn considered himself a beloved member of the family.
Eventually though, adventures, setting up Farshore, and a real life family of their own meant that Verrik and Larissa were unable to spend time with him. When Verrik and Larissa died, Maximilian passed into the possession of Lavinia, and although he knows this, he has never met Lavinia.
When Vanthus came to the vault, he was curt and dismissive of Maximilian, thinking of him as just another mindless construct. Maximilian was disappointed with Vanthus, and he knows that Lavinia will have to visit soon, and looks forward to this meeting with her.
He has come to think of her as something between a younger sister and a master, so he may seem overbearing or even impertinent with her, treating her as a close friend.